1. Field of the Invention
This invention relates generally to a membrane electrode assembly (MEA) for a proton exchange membrane fuel cell and, more particularly, to an MEA for a proton exchange membrane fuel cell, where the anode and cathode catalyst layers are deposited on a diffusion media layer and then over-sprayed with an ionomer layer, and where the catalyst layer is about the same size as the diffusion media layer.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus, are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and the cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer and a solvent. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). A stack of the MEAs make up the PEMFC. MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
It is known in the MEA art to coat the catalyst layer on the polymer electrolyte membrane. The catalyst layer may be deposited directly on the membrane, or indirectly applied to the membrane by first coating the catalyst on a decal substrate. Typically the catalyst is coated on the decal substrate as a slurry by a rolling process. The catalyst is then transferred to the membrane by a hot-pressing step. This type of MEA fabrication process is sometimes referred to as a catalyst coated membrane (CCM).
After the catalyst is coated on the decal substrate, an ionomer layer is sometimes sprayed over the catalyst layer before it is transferred to the membrane. Because both the catalyst and the membrane include the ionomer, the ionomer spray layer provides a better contact between the catalyst and the membrane, because it decreases the contact resistance between the catalyst and the membrane. This increases the proton exchange between the membrane and the catalyst, and thus, increases fuel cell performance. U.S. Pat. No. 6,524,736 issued to Sompalli et al., and assigned to the assignee of this invention, discloses a technique for making an MEA in this manner.
The decal substrate can be a porous expanded polytetrafluoroethylene (ePTFE) decal substrate. However, the ePTFE substrate is expensive and not reusable. Particularly, when the catalyst is transferred to the membrane on the ePTFE substrate, a certain portion of the catalyst or catalyst components remain on the ePTFE substrate. Additionally, the ePTFE substrate stretches, deforms and absorbs solvents making a cleaning step very difficult. Hence, every ePTFE substrate used to make each anode and cathode is discarded.
The decal substrate can also be a non-porous ethylene tetrafluoroethylene (ETFE) decal substrate. The ETFE decal substrate provides minimal loss of catalyst and ionomer to the substrate because virtually all of the coating is decal transferred. The substrate does not deform and can be reused. For both of these processes, the anode and cathode decal substrates are cut to the dimensions of the final electrode size, then hot-pressed to the perfluorinated membrane, and subsequently, the decal substrate is pealed off.
MEAs prepared by the above described decal substrate transfer processes have exhibited failure along the catalyst edge. Particularly, the membrane has been shown to tear adjacent to the outer edge of the catalyst layers on both the anode and cathode side of the MEA. This failure typically corresponds to the edge of the decal substrate during the hot-pressing step. Because the decal substrates are smaller in area than the membranes and have a thickness of about 3 mm, the decal substrate or active area section of the membrane would experience higher pressures than the remaining bare membrane areas during the hot-pressing step. This translates to a possible weakening of the membrane along the catalyst edges.
In another known fabrication technique, the MEA is prepared as a catalyst-coated diffusion media (CCDM) instead of a CCM. The diffusion media is a porous layer that is necessary for gas and water transport through the MEA. The diffusion media is typically a carbon paper substrate that is coated with a microporous layer, where the microporous layer is a mixture of carbon and Teflon. A catalyst ink is typically patched coated by a screen printing process on top of the microporous layer, and then compressed. A piece of bare perfluorinated membrane is sandwiched between two pieces of the CCDM with the catalyst sides facing the membrane, and then hot-pressed to bond the CCDM to the membrane.
However, this MEA fabrication process also suffers from membrane failure proximate the edge of the catalyst layer. Additionally, because the catalyst layer is smaller in area than the diffusion media and the membrane, there are areas where the diffusion media directly contacts the membrane outside of the catalyst layer. Therefore, gases being transported through the diffusion media react directly with the membrane instead of the catalyst, possibly causing combustion as a result of the interaction of hydrogen and oxygen, which also may damage the membrane.
Also, in the CCDM process, the ionomer in the catalyst tends to be adsorbed into the diffusion media because of its porosity resulting in less ionomer that is available to electrically couple the catalyst layer to the membrane when the CCDM is hot pressed to the membrane. Therefore, there is a reduction in MEA peformance.
In order to achieve low cost, efficient and durable fuel cells, the fabrication process of the known MEAs needs to be improved. The present invention provides a technique for fabricating MEAs that is simplified, and results in a more durable MEA than those MEAs known in the art.